This invention relates to superconductive materials and the use of such materials as infrared sensors. Metal-oxide ceramic superconductive materials were discovered circa 1987. Such materials conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen (77K or -196.degree. C.).
Materials exhibiting this property are called high T.sub.c superconductors, where T.sub.c is the transition temperature at or below which superconductivity is exhibited. A particularly useful high T.sub.c, superconductor is a yttrium-barium-copper oxide, such as YBa.sub.2 Cu.sub.3 C.sub.x, where x is 4 to 8 and preferably 7, which has a T.sub.c, of 90K or higher.
High T.sub.c superconductors have found many applications in industry, since they can be cooled by Liquid Nitrogen (LN), instead of the awkward and expensive liquid helium used in the past. These applications include, for example, Josephson devices, magnetic detectors formed of superconducting quantum interference devices (SQUIDS) and bolometers.
Bolometers, or radiation sensors, have heretofore been fabricated of high T.sub.c material. (See "High T.sub.c Superconducting Infrared Bolometric Detector", B.E. Cole, SPIE Vol. 1394. "Progress in High-Temperature Superconducting Transistors and Other Devices", (1990), pp. 126-138. A bolometer converts absorbed energy to a temperature rise and detects that temperature rise with a thermally sensitive element, such as a resistor The highest bolometer sensitivity is achieved by selecting detector materials with the highest temperature coefficient of resistance (TCR) and low resistivity. Metals have a low TCR in the range of 0.002/.degree. C. to 0.003/.degree. C. at room temperature and a somewhat higher TCR at (LN) temperatures. Semiconductors can have a much higher TCR, but have a very high resistance. High T.sub.c materials have TCRs in the order of 0.5/.degree. C. and the change in resistance at the transition edge is extremely abrupt, making them excellent candidates for bolometric devices.
Another factor of importance in bolometer performance is the thermal properties of the sensor. Both thermal mass and thermal conductance should be minimized within constraints of proper thermal time constant.
From the above, it may be seen that the sensitivity of radiation detectors is limited by the device operating temperature. Presently, room temperature detectors are in wide use while liquid helium devices are reserved for the most demanding applications. For radiation detection in the near IR spectrum, i.e., below about 20 .mu.m, LN detectors offer better performance than room temperature detectors and lower cost than helium-cooled devices LN-cooled detectors for wavelengths longer than 20 .mu.m are presently not available. Infrared detectors detect radiation by either a thermal process or a photon process. In detectors employing a photon process, incident radiation excites a carrier from a valence state into a conduction state. Sensitive semiconductor detectors relying on this process are used to detect near infrared radiation. Photons at longer wavelengths have lower energies and can only excite small energy transitions. But low energy transitions can also be excited by thermal energy. As a result, photon detection devices become much less sensitive at wavelengths beyond 20 microns and are drowned out by thermal energy. Such devices only work well if cooled at liquid helium temperatures The High -Tc detector, however, exhibits high performance at the critical temperature and can be used at longer wavelengths where the photon process detectors (i.e., MCT, photovoltaic, or photoconductive detectors) lose sensitivity.
Accordingly, a need exists for a radiation detector that will provide an LN-cooled alternative for high performance infrared radiation detection at longer wavelengths i.e., in the range of 20 .mu.m or higher.